1. Field of the Invention
The present invention relates generally to the operation of re-programmable non-volatile memory systems such as semiconductor flash memory, and more particularly, to the management of the interface between a host device and the memory.
2. Background
Conventional computer systems typically include several functional components. These components may include a central processing unit (CPU), main memory, input/output (“I/O”) devices, and mass storage devices. In conventional systems, the main memory is coupled to the CPU via a system bus or a local memory bus. The main memory is used to provide the CPU access to data and/or program information that is stored in main memory at execution time. Typically, the main memory is composed of random access memory (RAM) circuits. A computer system with the CPU and main memory is often referred to as a host system.
A host system interfaces with mass storage devices for example, non-volatile memory systems (may also be referred to as “flash device”, “flash” or “flash card” interchangeably throughout this specification) via an interface.
In an early generation of commercial flash memory systems, a rectangular array of memory cells were divided into a large number of groups of cells that each stored the amount of data of a standard disk drive sector, namely 512 bytes. An additional amount of data, such as 16 bytes, are also usually included in each group to store an error correction code (ECC) and possibly other overhead data relating to the user data and/or to the memory cell group in which it is stored. The memory cells in each such group are the minimum number of memory cells that are erasable together. That is, the erase unit is effectively the number of memory cells that store one data sector and any overhead data that is included. Examples of this type of memory system are described in U.S. Pat. Nos. 5,602,987 and 6,426,893. It is a characteristic of flash memory that the memory cells need to be erased prior to re-programming them with data.
Flash memory systems are most commonly provided in the form of a memory card or flash drive that is removably connected with a variety of hosts such as a personal computer, a camera or the like, but may also be embedded within such host systems.
Typically, a host system maintains a file directory and allocates file data to logical clusters. A host system that uses a logical interface for accessing data may be referred to as a legacy host system. The term host system in this context includes legacy flash memory card readers and digital cameras and the like.
In conventional systems, a host maintains a file system and allocates file data to logical clusters, where the cluster size is typically fixed. A flash device is divided into plural logical sectors and the host allocates space within the clusters comprising of a plurality of logical sectors. A cluster is a sub-division of logical addresses and a cluster map is designated as a file allocation table (“FAT”). The FAT is normally stored on a storage device itself.
In conventional systems, when writing data to the memory, the host typically assigns unique logical addresses to sectors, clusters or other units of data within a continuous virtual address space of the memory system. Like a disk operating system (DOS), the host writes data to, and reads data from, addresses within the logical address space of the memory system. A controller within the memory system translates logical addresses received from the host into physical addresses within the memory array, where the data are actually stored, and then keeps track of these address translations. The data storage capacity of the memory system is at least as large as the amount of data that is addressable over the entire logical address space defined for the memory system.
Other file storage systems (or formats) are being developed so that a host does not have to perform the file to logical address mapping. However, these new file systems may still have to be used with legacy host systems to read data.
Therefore, there is a need for a method and system that allows a host system to efficiently access files in a flash device that uses a non-traditional file storage format.